Different portraits of Anis Amri. Photograph: German federal police/AFP/Getty Images

It was only once the officers had taken his fingerprints and compared them with those of Anis Amri, the man suspected of killing 12 people at a Christmas market in Berlin on Monday, that they knew for sure. The Tunisian lying on the street in the Sesto San Giovanni district had a €100,000 (£85,000) bounty on his head.

It remains unclear why Amri, 24, embarked on a 1,000-mile journey from Germany to Italy in the days after the murderous truck attack.

Italy was the first European country in which he had set foot when he arrived on the continent six years ago. But if he had nurtured any hopes of it providing him with an escape route, he was quickly to be proved wrong. Just hours after arriving at Milan’s central train station, he was dead.

Amri was born in 1992 in the central Tunisian town of Oueslatia, in one of the country’s poorest regions. During his drifting youth, he became known to Tunisian law enforcement agencies for his drug use and later for stealing a truck.

He was, admitted his sister Najoua this week, “no angel”, even if he had nothing to do with terrorism. At the age of 17 or 18, in the chaotic days following the 2011 revolution, he did what many other African teenagers have done, and decided to leave.

“His father and I are disabled,” his mother, Nour el Houda Hassani, told Associated Press this week. “So he was forced to steal to emigrate and get … goods that young people dream of.”

With a five-year prison sentence hanging over him for the theft of the truck, Amri paid for an illegal boat journey to Europe. He travelled to Lampedusa, Italy, promising his his parents, four sisters and brother that he would make money and send it home.

Arriving on Sicily, having deliberately discarded his personal documents, he pretended to be an underage refugee. At the school he attended in Catania, Amri drew attention to himself for repeated incidents of petty theft and physical abuse.

He had also set fire to his housing on Lampedusa in an apparent protest over the authorities’ slowness in dealing with his asylum application. He received a four-year prison sentence.

According to various sources, it could be that Amri began to be radicalised during his time in prison on Sicily. He was certainly not a model detainee. Exhibiting threatening behaviour and appearing to want to stir rebellion, he spent three and a half years inside, being transferred between six different institutions in Catania and Palermo during that time.

“He was very aggressive and violent,” an official of the prison in Palermo where Amri was detained told the Guardian. “He tried to fight prison guards and other detainees. We registered all these act of violence while in prison and sent them to the police.”

After Amri’s release he was due to be deported to Tunisia. But the deportation failed because he had no documents. After a few weeks in detention in Caltanissetta pending deportation, he was released and made use of a legal loophole to leave Italy.

“When he got out, he called us to say that he could not find work, and he was leaving for another country,” his mother said, speaking from Oueslatia. That country was Germany.

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Amri arrived in Germany in July 2015, reportedly claiming to be a politically persecuted Egyptian, according to Ralf Jäger, the interior minister of the western German state of North Rhine-Westphalia. After that he lived between that state, Baden-Württemberg and Berlin, though since February he had mainly resided in the capital.

A short piece of mobile phone footage posted on Amri’s Facebook page showed him on the banks of the river Spree, looking carefree and appearing to enjoy Berlin like any other young newcomer might.

His asylum application was rejected this summer, stamped with the explanation that it was “offensichtlich unbegründet”, or obviously groundless, because of his inability to prove to the authorities that he was Egyptian.

Yet again, however, his deportation could not be carried out because he possessed no valid personal documents. He denied being Tunisian, and the Tunisian authorities initially refused to accept he was one of their citizens.

Eventually, in August, after much bureaucratic to-ing and fro-ing, the Tunisian authorities agreed to send him a replacement passport. The papers from Tunis that would have validated his deportation arrived in Germany two days after the Berlin attack. Under normal circumstances, he would have been extradited before the end of the year.

While in Germany, Amri had come to the attention of the intelligence agencies in various German states, according to Jäger. They had classified him as a “gefährder”, a person likely to threaten public safety, because of his links to the radical Islamist scene.

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Connections included the network that centred on a 32-year-old German Iraqi Salafist, Ahmad Abdulaziz Abdullah A, also known as Abu Walaa or “the preacher without a face”, who was arrested in November and denies charges he helped recruit for Islamic State. Amri also had contact with a known Turkish Islamic fundamentalist, Hasan C, who is believed to have radicalised young Muslims in the back of the Duisburg travel agency where he worked. Another contact was Boban S, a hate preacher from Dortmund.

For months, Amri was under investigation by German intelligence over suspicions he was planning a serious crime against the state. He was believed to have been organising a break-in to gain funds to help him buy automatic weapons, so that “very possibly he could commit an attack later on, together with accomplices which he intended to recruit”, according to Berlin’s department of public prosecution. There is also evidence he researched bomb-making techniques on the internet.

Months before that, according to the phone records of some of the Salafist preachers being monitored by German authorities, Amri is believed to have offered himself as a suicide bomber. But what they managed to catch him saying during tapped phone calls was apparently so encoded there was not enough evidence on which to base an arrest.

In conjunction with this investigation, Berlin’s justice department had monitored his movements between March and September, when the decision was made to halt the surveillance because it had produced no evidence other than that Amri was a small-time drug dealer in Görlitzer Park, southern Berlin’s notorious magnet for dealers and users.

In July he was involved in a knife attack in a row over drugs. But he went underground when police tried to question him.

On Friday it materialised he had still been under some form of surveillance just days before the attack, as footage came to light of him emerging in the central Berlin district of Moabit, from a mosque known to intelligence agents as a focal point of the radical Islamist scene.

A video in which Amri claims responsibility for the attack and pledges his allegiance to Isis was published on Friday afternoon. In it he called his attack vengeance for airstrikes against Muslims and said he wished to become a martyr.

For his family, it is likely to have come as a huge shock. “When I saw the picture of my brother in the media I couldn’t believe my eyes. I am in shock and can’t believe it’s him who committed this crime,” his brother Abdelkader said. His sister Najoua said that in his Facebook messages he was “always smiling and cheerful”.

His mother told Associated Press she had not seen any evidence during Amri’s phone calls that her son had become radicalised. She last spoke to him on Friday. But despite her belief in his innocence she was adamant that, if his role in Monday’s attack was confirmed, she would “renounce him before God”.